The Dewey Decimal System is a classification system used by libraries to organize books and other resources based on subject matter. It was developed by American librarian Melvil Dewey in 1876. The system assigns a unique number to each book, which corresponds to its subject matter. The system is divided into ten main categories, which represent the most common subject areas, and each category is further divided into subcategories. For example, category 500 is Natural Sciences and Mathematics, and subcategory 510 is Mathematics. The system is still widely used in libraries today, although some libraries have adopted other classification systems.